In semiconductor fabrication processes, the resolution of a photoresist pattern begins to blur at about 45 nanometer (nm) half pitch. To continue to use fabrication equipment purchased for larger technology nodes, multi-exposure methods have been developed.
Multiple exposure or multi-patterning technology (MPT) involves forming patterns on a single layer of a substrate using two or more different masks in succession. As long as the patterns within each individual mask comply with the relevant minimum separation distances for the technology node, the combination of patterns formed using the plural masks may include smaller separations than the minimum separation distance. MPT allows line segments, and in some cases, vertices (angles) to be formed of a vertical segment and a horizontal segment on the same mask. Thus, MPT provides flexibility and generally allows for significant reduction in overall IC layout.
MPT is a layout splitting method analogous to an M-coloring problem for layout splitting in graph theory, where M is the number of masks used to expose a single layer (and the number of exposures). For example, if two masks are to be used (double patterning, DPT), it is customary to refer to the patterns as being assigned one of two “color types”, where the color corresponds to a photomask assignment.
If a given pattern in a given layer cannot be assigned to a different mask from every adjacent pattern in the same layer that is closer than the minimum separation distance, the layout has an MPT conflict. The designer can resolve the MPT conflict without increasing the number of masks by making a change to the layout or inserting a stitch.